Foxp3-expressing regulatory T cells (T regs), a subset of CD4+ T cells that are essential for preventing autoimmunity, can also suppress anti-microbial immune responses, but their activity during tuberculosis is largely unexplored. Our preliminary data show that T regs expand and accumulate at sites of infection during tuberculosis and have the capacity to suppress immune mechanisms that provide protection. Importantly, we have also shown that both of these properties, expansion and suppression, are manifested preferentially by T regs that are specific for Mycobacterium tuberculosis (Mtb) antigens. We propose to investigate the influence of T regs in limiting protective immune responses during tuberculosis. In Aim 1 we will characterize the origin of T regs that respond during tuberculosis. We will determine whether they are derived from populations of pre-existing T regs and/or whether CD4+ effector T cells are induced to express Foxp3 and become regulatory cells in the setting of tuberculosis. In Aim 2 we will address the specificity of T regs in tuberculosis. The long-term consequences of Mtb-specific T reg suppression during the early stages of the immune response will be determined. MHC class II-restricted mycobacterial antigens recognized by T regs during tuberculosis will be identified, and we will determine whether they are the same, or different, as Mtb antigens recognized by effector T cells. In Aim 3 we will determine the effect of increasing the precursor frequency of Mtb-specific T regs on the functional activity of Mtb-specific effector T cells. The influence of T regs on effector T cell priming, trafficking, and cytokine production will be assessed. This proposal takes advantage of a wide variety of immunologic tools in the mouse model to accomplish its goals. Understanding generated by this proposal will be highly relevant to vaccine design and testing, and may suggest improved immunization strategies that circumvent T reg-mediated suppression of protective immune responses. PUBLIC HEALTH RELEVANCE: The factors that limit the immune system's ability to eradicate Mycobacterium tuberculosis, the causative agent of tuberculosis, are poorly understood. Here we test the idea that a subset of pathogen-specific T lymphocytes impairs the ability of the immune system to clear the bacteria. Understanding the role of these cells in suppressing immunity during tuberculosis has the potential to inform new and effective vaccine strategies that circumvent their activity and enhance pathogen clearance.